Etch Footing Budget
# Etch Footing Budget
An etch footing budget defines how much unremoved material an integration can tolerate at the base of a sidewall, and the evidence needed to measure and attribute it. It describes a lateral protrusion left at the bottom corner of an etched feature, where material resists clearing even after the field region nearby has cleared.
Footing is a bottom-of-sidewall phenomenon, the geometric counterpart to undercut rather than its opposite in every sense. Undercut removes material that the pattern never exposed; footing leaves material behind that the process was meant to clear, usually at a base corner where local chemistry, charging or shadowing favors survival.
## Locate the foot relative to the base corner
The foot sits specifically at the corner formed by the sidewall and the layer beneath it. A protrusion along the body of the sidewall, or a bulge mid-trench, describes a different geometry and belongs under a different term even when it also represents incomplete clearing.
Common contributors include passivation that accumulates preferentially at the corner, ion-angle shadowing that reduces local ion flux at the base, and charging effects that deflect ions away from a narrow corner before the field region nearby has fully cleared. Which mechanism applies depends on the chemistry, feature aspect ratio and the specific stack, not on the appearance of the foot alone.
## Separate footing from neighboring sidewall-base effects
| Observation | What it describes | What it does not establish |
|---|---|---|
| Footing | Unremoved material at the sidewall base corner | A loss of material anywhere along the sidewall |
| Microtrenching | A localized deeper groove near the sidewall base | Any retained or unremoved material |
| Undercut | Lateral loss beneath a mask or stop-layer edge | Material surviving at the base of the feature |
| Notching | Lateral loss concentrated at one interface during a step | A base-corner protrusion |
These four terms describe adjacent but distinct locations and material states around the same general region of a feature. A report that groups them as one generic "base defect" cannot be checked against a budget written for any one of them specifically.
## Build the footing budget around the clearing requirement
Fix the clearing plane the process is meant to reach before reporting a foot as present or absent. A foot measured against the wrong reference surface, or measured only where the field region is easiest to inspect, does not represent the feature population the budget is meant to protect.
Report foot height and width separately rather than a single combined residue number, since a short wide foot and a tall narrow one can have different consequences for contact landing, silicide formation or subsequent fill, even at similar apparent severity in a plan-view image.
## Qualify the consequence, not only the presence
A foot that remains fully within the intended clearing margin may not affect the device, while a smaller foot at a critical electrical interface can still cause a failure. Evaluate the measured foot against the specific downstream step it threatens before treating any nonzero residual as a defect.
A footing budget ties a measured base-corner residual to a fixed clearing plane and the specific downstream step it threatens, rather than to a visual impression of incomplete clearing. Mechanism attribution, dimensional measurement and functional consequence remain separate questions that each need their own evidence.
## Sources and scope
- [Lam Research — Plasma Etch Fundamentals](https://www.lamresearch.com/plasma-etch): process description of ion-angle and charging effects that influence base-corner clearing in plasma etch.
- [Oxford Instruments — Etch Process Control](https://plasma.oxinst.com/media-centre/blog/etch-process-control): discussion of passivation and selectivity effects near feature base corners.
Original diagrams and explanatory synthesis. No universal footing limit, mechanism attribution or qualified process fix is implied.